Hot melt thermoplastic adhesives have been widely used in industry for adhering many types of products, and are particularly useful in applications where quick setting time is advantageous. One application for hot melt adhesives which has been of considerable interest in recent years is the bonding of non-woven fibrous material to the backing sheet of hygienic articles such as disposable diapers, incontinence pads and similar articles.
In the prior art, the manufacture of disposable diapers, for example, has involved the application of multiple, parallel beads of pressure sensitive hot melt adhesive onto the backing sheet of the diaper so as to adhere the backing sheet to the non-woven, absorbent pad of the diaper. To ensure a good bond is obtained between the non-woven pad and backing sheet, and an acceptable visual appearance of the resulting product, the adhesive beads must be accurately positioned along the backing sheet and formed in a uniform width.
One apparatus commonly employed in the manufacture of disposable diapers and other hygienic articles is a metering gear head having a plurality of spaced discharge orifices, each supplied with adhesive from a gear pump, which are positioned relative to the backing sheet to apply parallel beads of adhesive thereto for subsequent attachment to a non-woven absorbent pad. The primary advantages of metering gear heads are that they provide for precise control of the quantity of adhesive dispensed, and accurately locate the adhesive beads on the backing sheet of the diaper, so that the resulting product has multiple adhesive beads of uniform size, width and spacing.
Despite these advantages, metering gear heads also present problems in the manufacture of disposable diapers and other hygienic articles which employ a backing sheet formed of polyethylene, polypropylene, polyurethane or similar materials. The specific heat of the hot melt adhesive discharged in the form of beads from metering gear heads onto the backing sheet has the potential to burn through the backing sheet or at least distort the material and produce an unacceptable product. While this problem can be reduced or overcome by increasing the thickness of the backing sheet, the added thickness increases the cost of the backing sheet. In addition, the adhesive beads discharged onto one side of the backing sheet are readily visible from its outer side which detracts from the appearance of the finished product.
Attempts have been made in the prior art to avoid the problem of burn through or distortion of the backing sheet of hygienic articles, caused by the heat of the hot melt adhesive beads applied thereto, without increasing the thickness of the material forming the backing sheet. In one prior art method, the hot melt thermoplastic adhesive is formed in elongated, thin strands or fibers which are deposited atop the non-woven layer of the article and then the non-woven layer is bonded to the backing sheet. The specific heat of the thin, elongated strands or fibers of adhesive is less than that of the relatively thicker and wider adhesive beads discharged from metering gear heads, and thus the problem of burn through or distortion of the backing sheets of hygienic articles is reduced so that thinner materials can be employed in the manufacture of the backing sheets. In addition, the thin fibers or strands of adhesive are essentially invisible through the backing sheet which produces a more aesthetically acceptable finished product.
Prior art spray devices capable of producing such elongated adhesive strands or fibers include a nozzle formed with an adhesive discharge opening and one or more air jet orifices through which a jet of air is ejected. A bead of adhesive is discharged from the adhesive discharge opening in the nozzle which is then impinged by the air jets to attenuate or stretch the adhesive bead forming thin fibers for deposition onto the substrate. Examples of spray devices of this general type are disclosed in U.S. Pat. Nos. 2,626,424 to Hawthorne, Jr.; 3,152,923 to Marshall et al; and, 4,185,981 to Ohsato et al.
Prior art spray devices of the type described above are also capable of accurately controlling the location of the spray patterns of elongated adhesive strands or fibers discharged onto the substrate, and these spray patterns are nearly invisible on the finished product. In the devices such as disclosed, for example, in the U.S. Pat. No. 2,626,424 patent to Hawthorne, Jr. and the U.S. Pat. No. 4,185,981 to Ohsato et al, the air jets are directed substantially tangent to the adhesive bead ejected from the discharge opening in the nozzle which rotates or twists the elongated adhesive fibers in a relatively tight, compact spiral spray pattern for application onto the substrate.
In a commercial disposable diaper production line, for example, conventional metering gear heads apply 32 or more individual, parallel beads of adhesive onto a moving backing sheet which is subsequently cut along both its width and length to form individual diapers. In applications of this type, prior art spray systems for spraying spiral patterns of adhesive fibers or strands, such as described above, are not entirely suitable. A separate nozzle is required in such systems to produce a spiral spray pattern adhesive fibers at each of the 32 locations where an adhesive bead had been applied by prior art metering gear heads. Whereas each discharge outlet which produces an adhesive bead in a metering gear head is directly supplied with adhesive from a separate supply line connected to a gear pump, the nozzles of prior art spiral spray systems are not directly connected to a pump. Instead, a number of nozzles are supplied with adhesive from a common supply line connected to a pump. Depending upon the pressure drop in such common supply lines from nozzle to nozzle, the quantity of adhesive discharged from each nozzle can vary. That is, the quantity of adhesive discharged from a given nozzle decreases as the pressure in the supply line at such nozzle decreases.
In order to produce an acceptable bond between the backing sheet and non-woven layer of a hygienic article, a predetermined minimum quantity of adhesive must be applied to the backing sheet by each nozzle of such prior art spiral spray systems. Although the problem of pressure drop within the adhesive supply lines of such spiral spray systems can be overcome by increasing the pump pressure so that the pressure in the supply lines at each nozzle location results in the discharge of at least the minimum required quantity of adhesive from each nozzle, this increase in pump pressure results in an overall increase of the total amount of adhesive discharged onto the backing sheet. This is because the pressure in the supply lines at some of the nozzles will be greater than that required to discharge the minimum amount of adhesive from such nozzles, and thus too much adhesive is applied to the backing sheet from these nozzles. This wastes adhesive and adds to the cost of the finished product.